Pulse transformer for firing thyristors

ABSTRACT

An improved structure of a pulse transformer for simultaneously firing all the gates of a multiplicity of series-connected or series- and parallel-connected thyristors as is the case with a converter in dc transmission equipment, having a greater insulation strength and a good resistivity to external shocks. Specifically, since the converter used at high tension renders the line voltage of each thyristor higher, a molded insulating cylinder is disposed between a primary conductor and secondary conductor units of the transformer, whereby potential distributions between the secondary transformer units and the insulating cylinder and between the secondary transformer units adjacent to one another can be improved by providing conductive layers embedded in the insulating cylinder, and the pulse transformer can be made of a dry tape and small in size.

United States Patent Isogai et al.

PULSE TRANSFORMER F OR FIRING THYRISTORS inventors: Tokio lsogai; Eiichi Isikawa; l-lisashi Yuza; Takasi Tahara; Takao Miyashita; Norio lkemoto, all of Hitachi, Japan Assignee: Hitachi, Ltd., Tokyo, Japan Filed: Nov. 2, 1970 Appl. No.: 86,028

Field ofSearch ..321/8 R, 11,27 R; 336/70, 336/84, 174, 175, 184, 192, 229

References Cited UNITED STATES PATENTS 3,398,349 8/1968 Evans,Jr.etal ..32l/11X 3,028,568 4/1962 Camilli ..336/70 [15] 3,654,543 [451 Apr. 4, 1972 Toulemonde et al.. ..321/8 Hartmann ..336/70 X Primary Examiner-A. D. Pellinen Attorney-Craig, Antonelli & Hill ABSTRACT An improved structure of a pulse transformer for simultaneously firing all the gates of a multiplicity of series-connected or seriesand parallel-connected thyristors as is the case with a converter in do transmission equipment, having a greater insulation strength and a good resistivity to external shocks. since the converter used at high tension renders the line voltage of each thyristor higher, a molded insulating cylinder is disposed between a primary conductor and secondary conductor units of the transformer, whereby potential distributions between the secondary transformer units and the insulating cylinder and between the secondary transformer units adjacent to one another can be improved by providing conductive layers embedded in the insulating cylinder, and the pulse transformer can be made of a dry tape and small in size.

14 Claims, 6 Drawing Figures Patented April 4, 1972 3,654,543

5 Sheets-Shoot 5 FIG. 3

INVENTORS 2, QM WA Nfl ATTORNEYS Patented April 4, 1972. 3,654,543

5 Shoots-Shut 4.

INVENTORS BY QM, WVMLUL ATTORNEYS Patented April 4, 1972 5 Shoots-Shut E,

INVENTORS Tovuo 3:50am I Emu-H ICEAKHWH' msasm vuza, TFIKRSU THHHRH, TF! K90 MHRsHlTR Nome nuzmoro BY AM,W 1M2 ATTORNEYS 1 PULSE TRANSFORMER FOR FIRING TI-IYRISTORS BACKGROUND OF THE INVENTION 1. Field of the Invention This invention relates to an improvement in pulse transformers for firing serially connected multi-stage thyristors having a high breakdown voltage, for example, serially connected thyristors in a thyristor ac-dc converter for use in high voltage dc transmission.

Description of the Prior Art When serially-connected thyristors are used in an ac-dc converter for high voltage dc transmission use, they may be operated either as a forward converter (rectifier) or as a reverse converter (inverter) by controlling the firing phase of the thyristors in respective stages.

When such a converter is to be operated at high voltage e. g., a hundred thousand to several hundred thousand volts, a single thyristors cannot bear a reverse voltage or forward voltage during the nonconductive period at such magnitude and so an appropriate number of thyristors must be used in series in such a converter.

One important problem when using serially connected thyristors is how to fire all the thyristors at the same instant in response to a fire signal. The prior art includes as firing systems for providing multiple fire signals such systems wherein fire signals are produced from a light pulse using phototubes, and a system wherein fire signals are produced through secondary windings wound on an iron core provided with a common primary winding, as is disclosed in British Pat. No. 1,130,925.

Compared with the former firing system utilizing a light pulse, the latter system of a pulse transformer has an advantage of using an electric pulse in that the manufacture of the firing circuit is easier. Theoretically, a desired number of firing circuits can be made simply by winding the desired number of secondary windings on a continuous iron core and all the thyristors can be fired at the same instant by supplying a pulse current to a primary winding on the core. However, in such a pulse transformer, there is the need for strengthening the insulation against ground potential in the respective transformer units, each being formed for one thyristor, as going from the lower to the higher voltage side of the thyristors. Due to this consideration, an oil-immersed type is usually employed in which respective transformer units and the primary winding, etc. are immersed in an oil vessel.

However, pulse transformers of the oil-immersed type are relatively large in size and need, monitoring of insulating oil, therefore the maintenance and inspection is troublesome.

Based on these facts, there has been proposed a dry type insulation in such a pulse transformer in which insulation is provided by an insulating cylinder disposed between transformer units and a primary conductor. But, in such a system, corona discharges often occur between the insulating cylinder and the primary conductor, between the insulating cylinder and the respective transformer units, and between adjacent transformer units, and at the end portions of the insulating cylinder, etc. Thus, the dry type insulation is considered very difficult to achieve.

Further, in a pulse transformer of the dry type insulation, the insulating distance between each pair of adjacent transformer units should be selected large enough to prevent any occurrence of corona discharge thereat, and thereby the total height of the pulse transformer becomes larger than that of piled thyristors, contradictory to the requirement for compactness.

Yet further, for higher voltages, the number of transformer units inevitably increases, resulting in an increase in the total height which affects the structural strength against external mechanical shocks, such as earthquakes.

SUMMARY OF THE INVENTION An object of this invention is to provide a pulse transformer of a dry insulation type in which the occurrence of a corona discharge can be effectively prevented.

Another object of this invention is to provide a pulse transformer in which the insulating distance between each pair of adjacent transformer units is decreased and the surface potential gradient in an insulating cylinder is uniformalized by uniformly sharing the absolute potential, thereby decreasing the total height of the structure.

A further object of this invention is to provide a pulse transformer having a large number of transformer units in which the manufacture of a long insulating cylinder is easy.

Another object of this invention is to provide a pulse transformer which is hard to get damaged by external mechanical shocks.

According to a feature of this invention, a conductive layer is formed on the inner peripheral portion of an insulating cylinder and kept at an equal potential with a primary conductor extending through the cylinder, other conductive layers are formed at least in the surface portions of transformer units on the high voltage side facing said insulating cylinder and in the corresponding surface portions of said insulating cylinder with respective conductive connections therebetween, and further, the conductive layers in the insulating cylinder are coupled electrostatically to one another.

According to another feature of the invention, an insulating cylinder in a pulse transformer is divided into plural portions which are piled one on another. The end portions of the divided cylinder portions facing each other have diverging and converging surfaces to fit one another. In these end portions of the insulating cylinder, conductive layers electrostatically coupled in a radial direction are provided with their ends offset so as to form an equipotential surface perpendicular to said facing surfaces.

When a transformer is of higher voltage, the number of transformer units increases, hence the height of the insulating cylinder becomes higher, and accordingly the structure becomes weaker against external shocks.

To solve this problem, it may be considered to independently support portions of the insulating cylinder. However, the surface potential of the insulating cylinder becomes higher going further away from its base which is kept at ground potential, therefore this support system is undesirable from the insulation viewpoint.

A larger insulating support is necessary to support an object body with enough insulation strength, contradictory to the requirement for reduction in size. Further, to solidly fix an insulating cylinder on a mount invites direct affects from external forces.

According to an embodiment of this invention, an insulating cylinder is supported by a frame through an upper vessel connected at the top portion of the cylinder and resilient means for receiving said vessel. Thus, external forces are absorbed by said resilient means.

Other features of this invention will be readily apparent from the following detailed description on certain preferred embodiments taken in conjunction with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a schematic electrical connection diagram of an example of a circuit for firing thyristors in a high voltage thyristor converter using a pulse transformer;

FIG. 2 is a partially cross-sectional front view of an embodiment of a pulse transformer for firing thyristors according to the invention;

FIG. 3 is an elevated partial cross-section of an insulating cylinder and transformer units of the pulse transformer shown in FIG. 2;

FIG. d is an explanatory front view of the insulating cylinder for showing the surface potential distribution of the cylinder;

FIG. 5 is a cross-sectional view of an alternative embodiment of the transformer unit; and

FIG. 6 is a cross-section along line VI VI of FIG. 3.

DESCRIPTION OF THE PREFERRED EMBODIMENTS FIG. 1 shows a circuit for firing a thyristor converter in which thyristors SR SR,, and SR, are serially connected to have two lead-out terminals L and L and are fired by pulse signals applied to respective gates G G,, and G,,. Voltage dividing circuitry B B,, and B, is usually formed of resistors R and capacitors C for uniformalizing the applied voltages on respective thyristors. Zener diodes Z 2,... are connected between the gate and the cathode of the respective thyristors through rectifiers D D and D An input pulse current is generated in a pulse current generator PG and is allowed to flow through a primary conductor W conductively connected to this pulse generator PG. At positions of this primary conductor W a multiplicity of superposed annular iron cores RC RC,, and RC are disposed, which are wound with respective secondary windings W W and W The outputs of the secondary windings are rectified by the respective corresponding rectifiers and supplied to the respective thyristor at a predetermined voltage through the respective Zener diode, to fire the respective thyristors simultaneously.

FIG. 2 shows an overall structure of a pulse transformer according to the invention inwhich an insulating support frame 2 is fixed on a base 1 with bolts 2a. The insulating support frame 2 is formed of a plurality of insulating stays 3 standing vertically on the base 1, insulating transverse bars 4 connecting and reinforcing the insulating stays 3, and insulating cross bars 5 connecting and reinforcing the transverse bars 4 and the insulating stays 3. A receiving seat 7 for a pulse transformer 6 is provided on the insulating support frame 2. The pulse transformer comprises a primary conductor 8, an insulating cylinder 10 around the primary conductor 8, and a plurality of transformer units 11 fitted around the insulating cylinder 10. The insulating cylinder 10 is formed by wrapping a sheet of insulating cloth, such as glass cloth or a tetron (polyethylene terephthalate) cloth, around a central tube 9 through which the primary conductor 8 penetrates, adding epoxy resin thereon, and thermally setting the structure.

In the embodiment shown in FIG. 2, two pulse transformer elements 6A and 6B are disposed in side-by-side relationship (but series in connection) and are fitted in metal supports 13 through flanges 12 provided on the upper ends of the insulating cylinders 10. The metal supports 13 are fixed to an upper vessel 14, for example through bolts 15.

The upper vessel is filled with an insulating medium 16, eg insulating oil. The upper ends of the respective insulating tube 10, the central tube 9 and the primary conductor 8 are disposed in this insulating oil 16 and both the primary conductors 8 are connected together by a conductor 17. For facilitating the connection of the primary conductor 8 with the connection conductor 17, a cover 18 is provided in the upper vessel 14.

For absorbing thermal expansion or contraction of the insulating oil 16 in the upper vessel 14, an inert gas 19 may be sealed in the upper portion of the vessel 14 and a well-known pressure absorbing means which can absorb the pressure variation of the inert gas 19 may be provided according to need. The pulse transformers 6A and 6B received in the upper vessel 14 are resiliently supported on said receiving seat 7. As this resilient support means, combinations of a spring 20 and a guide 21 may be disposed between the upper vessel. 14 or the metal supports 13 and the receiving seat 7.

At the lower end portions of the insulating cylinders 10, a lower vessel 23 is attached through flanges 22, This lower vessel is filled with insulating oil 16 and is continuously supplied with insulating oil 16 in the upper vessel 14 through the inner space of the central tube 9. Bolts 24 fix the flanges 22 to the cover 25 of the lower vessel 23 and bolts 26 fix the cover 25 to the lower vessel 23. The lower ends of the pulse transformers 6A and 6B are introduced into this lower vessel 23 in a manner similar to the upper vessel and the ends of the primary conductor 8 disposed in the lower vessel are conductively connected to a pair of terminals 27a and 27b by way of lead-out conductors 28a and 28b. A pulse current generator is conductively connected to these terminals 27a and 27b.

0n the bottom of the lower vessel 23, a projection comprising a rod 29 and a flange 30 is fixed to rest on a lower guide means 33 including a spring 32 in a cylinder 31 mounted on the base 1. Thus the pulse transformer 6 is elastically supported by said upper elastic support means and said lower guide means 33 to protect it from external shocks.

The primary conductor 8 may be dispensed with by forming the central tube 9 from a good conductor to work also as a primary conductor. The structures of said insulating cylinder 10 and said transformer units 11 will be described in detail hereinafter referring to FIGS. 3 to 6. In case of using a central tube 9 also as a primary conductor, hollow insulating cylinders 10a and 10b, vertically divided, surround the central tube 9. Conductive layers 34 and 35 are disposed on the inner periphery of the insulating cylinder 10a and 10b along an imaginary longitudinal axis and are conductively connected to said central tube 9, respectively, so as to keep the oil gap between the central tube 9 and the insulating cylinders 10a, 10b at the same potential. A transformer unit is fitted around an insulating cylinder, or 1012, and comprises a magnetic iron core 37 made of molybdenum permalloy, etc., a secondary winding 38 wound on the magnetic iron core 37, a conductive layer 39 formed at least on such surface portion of the secondary winding 38 which faces the insulating cylinder 10a or 10b, and an insulating body 40 for insulating and molding the secondary winding 38 and the conductive layer 39. Said conductive layer 39 may be either embedded in the insulating body 40 as is shown in FIGS. 3 and 4, or adhered to the surface of the insulating body 40 as is shown in FIG. 5. A conductive layer 41 is formed in the surface portion of the insulating cylinder 10a or 10b corresponding to and facing each transformer unit 11 and is conductively connected to the conductive layer 39 of the corresponding transformer unit 11 by a conductor 42 to keep the gap between the insulating cylinder 10a or 10b and the transformer unit 11 at a uniform potential. In the upper and lower end portions of each insulating cylinder, a plurality of conductive layers 43a and 44a are disposed to form a voltage dividing capacitor to uniformalize the electric field in the end portions of the insulating cylinder. Conductive layers 45 electrostatically couple respective pairs of adjacent conductive layers 41 in a longitudinal direction, the detailed structure of which will be described with reference to FIG. 4.

FIG. 4 shows a structure of a voltage dividing capacitor 46 formed in the surface portion of the insulating cylinder 10a at a position intermediate a pair of adjacent transformer units 11a and 11b. Said voltage dividing capacitor 46 comprises conductive layers 47a, 47b and 47: disposed offset to each other between the conductive layers 41 corresponding to a pair of adjacent transformer units 11a and 11b.

The number of the conductive layers forming said voltage dividing capacitor 46 may be arbitrarily selected. And the innermost conductive layer 47c of the capacitor 46 is conductively connected to the conductive layer 41.

By the above structure, the potential at the surface portion of each insulating cylinder can be determined and lines of electric force I between each pair of adjacent transformer units are distributed perpendicularly from the respective conductive layers of the capacitor 46 to the surface of the insulating cylinder 10a. Thus, the potential distribution in the spacing between said transformer units can be uniformalized, rationally enabling a reduction in the size of this spacing.

The insulating cylinder on which respective transformer units are to be fitted may be composed of a single continuum. However, when the voltage in the thyristor circuit is relatively high, the insulating cylinder may be divided into two or more portions to facilitate the manufacture thereof, as is the case with the embodiment of FIG. 3.

In the case of a divided insulating cylinder, consideration should be given not to allow the occurrence of a corona discharge at the jointed portions of the divided cylinder. In FIG. 3, for example, the upper end of an insulating cylinder b has a tapered projection and the lower end of another insulating cylinder 10a has a cone-shaped dent to fit on the above projection. Further, the ends of respective conductive layers of the voltage dividing capacitor 43a at the end of the insulating cylinder 10a and those of the capacitor 44a at the end of the insulating cylinder 1012 are correspondingly offset to arrange for an equipotential surface between said ends of the two capacitors to perpendicularly cross the facing end surfaces of the two cylinders, thereby setting the electric lines of force so that they extend along and within the gap g between the two cylinders. Thus, the insulation in the junction portion is strengthened. Here, the innermost electrodes of the voltage dividing capacitors 43a and 44a are maintained at an equal potential through the conductive layers 3d, 35 and the central tube 9, and the outermost electrodes are conductively connected to each other by a connection wire 48. The gap g between said insulating cylinders 10a and 10b is continuously open at the inner periphery to the oil gap 36 to introduce insulating oil and is hermetically sealed at the outer periphery with a packing 49.

In the above embodiments, one secondary winding is wound in each transformer unit. However, a plurality of secondary windings may be wound on each unit as is shown in FIG. 6. In the figure, secondary windings 38a, 38b, 38c and 38d are separately disposed on a magnetic iron core 37 and respectively connected to lead-out terminals 50. The transformer unit 11 is coupled to an insulating cylinder 10b through coupling means 51a to 51d. It will be apparent that the above structure enables a reduction in the number of transformer units and hence the overall size of the pulse transformer is also reduced.

Further, as is shown in FIG. 2, the primary conductor may be folded to reduce the height. In this structure, connection to the serial thyristors is alternately made from the right half portion and the left half portion to uniformalize the electric field in the transformer.

As is described above, in a pulse transformer for firing high voltage thyristors, the threshold voltage for allowing corona discharge between the primary conductor and the insulating cylinder, between the insulating cylinder and the transformer units, between the transformer units, etc. can be increased according to this invention, thereby enabling a reduction in the thickness of the insulating cylinder and/or the size of the transformer unit. Further, the surface potential distribution in the insulating cylinder can be arranged in preferable form to improve the voltage resistant property of the surface. Thus, the height of the pulse transformer can be reduced.

Therefore, according to this invention, a smaller pulse transformer of the dry type having a larger insulation strength can be provided for use in firing high voltage thyristors.

We claim:

l. A pulse transformer for firing thyristors comprising a pri mary conductor conductively connected to a pulse generator, an insulating cylinder through which the primary conductor extends, and a plurality of transformer units fitted around said primary conductor through said insulating cylinder therebetween, each of said transformer units including a magnetic iron core, a secondary winding wound on said iron core and connected to the gate of a thyristor and an insulating body for insulating said secondary winding,

a first conductive layer disposed in the inner peripheral portion of the insulating cylinder and kept at the same potential as the primary conductor;

second conductive layers disposed in the inner peripheral portions of said insulating bodies of at least those transformer units which are positioned on the higher voltage side, facing said insulating cylinder;

third conductive layers disposed in those surface portions of said insulating cylinder which face said second conductive layers, said second conductive layers being electrically connected to said third conductive layers, respectively.

2. A pulse transformer for firing thyristors as defined in claim 1, wherein each of said second conductive layers is formed in the inner peripheral portion and in the upper and lower surface portions of said insulating body.

3. A pulse transformer for firing thyristors as defined in claim 1, wherein said second conductive layers are embedded in said insulating bodies.

4. A pulse transformer for firing thyristors as defined in claim 1, wherein each of said third conductive layers extends beyond the upper and lower surface of the corresponding transformer unit.

5. A pulse transformer for firing thyristors as defined in claim I, wherein said third conductive layers are disposed adjacent the surface of said insulating cylinder.

6. A pulse transformer for firing thyristors as defined in claim ll, wherein said primary conductor is formed in a hollow cylindrical shape with a cable penetrating through said hollow portion.

7. A pulse transformer for firing thyristors as defined in claim 1, wherein each of said transformer units includes a plurality of electrically separated secondary windings disposed on an annular magnetic iron core with equal spacings, and terminals corresponding to the respective secondary windings and projected from said insulating body.

8. A pulse transformer for firing thyristors as defined in claim I, further comprising a mounting base, a support seat mounted on said mounting base, an upper and a lower vessel attached to the upper and the lower end portion of said insulating cylinder, resilient means for resiliently supporting said upper vessel on said support seat, and guide means for guiding said vessel at a position in said mounting base.

9. A pulse transformer for firing thyristors comprising a primary conductor conductively connected to a pulse generator, an insulating cylinder through which said primary conductor penetrates, and plurality of transformer units fitted around said primary conductor with said insulating cylinder disposed therebetween, each of said transformer units including a magnetic iron core, a secondary winding wound around said iron core and connected to the gate of a thyristor and an insulating body for insulating said secondary winding, a plurality of mutually independent first conductive layers each disposed in those surface portions of said insulating cylinder which face a respective transformer unit, said first conductive layers being spaced with a gap in the axial direction of said insulating cylinder; and a plurality of second conductive layers embedded in said insulating cylinder in groups corresponding to and electrostatically coupling with each of said first conductive layers, said second conductive layers being offset with respect to each other in both the radial and axial directions, one end portion of said second conductive layers being positioned in radial alignment with spaced points in the gaps between said first conductive layers and being spaced and staggered from each other with a predetermined interval in the axial direction of said insulating cylinder.

10. A pulse transformer for firing thyristors as defined in claim 9, wherein each of said first conductive layers are kept at an equal potential with that of the surface portion of the insulating body of the corresponding transformer unit, and the innermost layer of said second conductive layers being electrically connected to the nearest first conductively layer so as to form a series of electrostatic capacitive couplings in the axial direction of said insulating cylinder.

11. A pulse transformer for firing thyristors as defined in claim 10, wherein said primary conductor is formed of nonmagnetic material in the form of an integral hollow cylinder through which an energizing cable is inserted, thereby physically reinforcing said insulating cylinder.

12. A pulse transformer for firing thyristors comprising a primary conductor conductively connected to a pulse generator, an insulating cylinder divided into a plurality of portions in axial direction, said primary conductor extending through said insulating cylinder, and a plurality of transformer units fitted around said primary conductor through said insulating cylinder therebetween, each of said transformer units including a magnetic iron core, a secondary winding wound on said iron core and connected to the gate of a thyristor and in insulating body for insulating said secondary winding the improvement comprising:

a first conductive layer disposed in inner peripheral portion of said insulating cylinder and kept at an equipotential to that of said primary conductor;

second conductive layers disposed in the inner peripheral portions of said insulating bodies of at least those transformer units which are positioned on the higher voltage side, facing said insulating cylinder;

third conductive layers disposed in those surface portions of said insulating cylinder which face said second conductive layers, said second and third conductive layers being electrically connected respectively;

said insulating cylinder portions having a converging and a diverging end surface;

fourth conductive layers disposed in each facing end portion of said divided cylinder portions and electrostatically coupled to respective one of said third conductive layers, the end lines of said fourth conductive layers being offset in a radial direction to form an equipotential surface substantially perpendicular to the facing end surfaces of said insulating cylinder portion.

13. A pulse transformer for firing thyristors as defined in claim 12, further comprising gaps between said primary conductor and said insulating cylinder, and between each pair of adjacent cylinder portions, said gaps being continuous, and a highly insulating medium filled in said gaps.

14. A pulse transformer for firing thyristors as defined in claim 13, further comprising an upper and a lower vessel respectively attached at the upper and the lower end of said insulating cylinder and an insulating medium contained in said vessel, said gaps being continuous to said upper and lower vessel. 

1. A pulse transformer for firing thyristors comprising a primary conductor conductively connected to a pulse generator, an insulating cylinder through which the primary conductor extends, and a plurality of transformer units fitted around said primary conductor through said insulating cylinder therebetween, each of said transformer units including a magnetic iron core, a secondary winding wound on said iron core and connected to the gate of a thyristor and an insulating body for insulating said secondary winding, a first conductive layer disposed in the inner peripheral portion of the insulating cylinder and kept at the same potential as the primary conductor; second conductive layers disposed in the inner peripheral portions of said insulating bodies of at least those transformer units which are positioned on the higher voltage side, facing said insulating cylinder; third conductive layers disposed in those surface portions of said insulating cylinder which face said second conductive layers, said second conductive layers being electrically connected to said third conductive layers, respectively.
 2. A pulse transformer for firing thyristors as defined in claim 1, wherein each of said second conductive layers is formed in the inner peripheral portion and in the upper and lower surface portions of said insulating body.
 3. A pulse transformer for firing thyristors as defined in claim 1, wherein said second conductive layers are embedded in said insulating bodies.
 4. A pulse transformer for firing thyristors as defined in claim 1, wherein each of said third conductive layers extends beyond the upper and lower surface of the corresponding transformer unit.
 5. A pulse transformer for firing thyristors as defined in claim 1, wherein said third conductive layers are disposed adjacent the surface of said insulating cylinder.
 6. A pulse transformer for firing thyristors as defined in claim 1, wherein said primary conductor is formed in a hollow cylindrical shape with a cable penetrating through said hollow portion.
 7. A pulse transformer for firing thyristors as defined in claim 1, wherein each of said transformer units includes a plurality of electrically separated secondary windings disposed on an annular magnetic iron core with equal spacings, and terminals corresponding to the respective Secondary windings and projected from said insulating body.
 8. A pulse transformer for firing thyristors as defined in claim 1, further comprising a mounting base, a support seat mounted on said mounting base, an upper and a lower vessel attached to the upper and the lower end portion of said insulating cylinder, resilient means for resiliently supporting said upper vessel on said support seat, and guide means for guiding said vessel at a position in said mounting base.
 9. A pulse transformer for firing thyristors comprising a primary conductor conductively connected to a pulse generator, an insulating cylinder through which said primary conductor penetrates, and plurality of transformer units fitted around said primary conductor with said insulating cylinder disposed therebetween, each of said transformer units including a magnetic iron core, a secondary winding wound around said iron core and connected to the gate of a thyristor and an insulating body for insulating said secondary winding, a plurality of mutually independent first conductive layers each disposed in those surface portions of said insulating cylinder which face a respective transformer unit, said first conductive layers being spaced with a gap in the axial direction of said insulating cylinder; and a plurality of second conductive layers embedded in said insulating cylinder in groups corresponding to and electrostatically coupling with each of said first conductive layers, said second conductive layers being offset with respect to each other in both the radial and axial directions, one end portion of said second conductive layers being positioned in radial alignment with spaced points in the gaps between said first conductive layers and being spaced and staggered from each other with a predetermined interval in the axial direction of said insulating cylinder.
 10. A pulse transformer for firing thyristors as defined in claim 9, wherein each of said first conductive layers are kept at an equal potential with that of the surface portion of the insulating body of the corresponding transformer unit, and the innermost layer of said second conductive layers being electrically connected to the nearest first conductively layer so as to form a series of electrostatic capacitive couplings in the axial direction of said insulating cylinder.
 11. A pulse transformer for firing thyristors as defined in claim 10, wherein said primary conductor is formed of non-magnetic material in the form of an integral hollow cylinder through which an energizing cable is inserted, thereby physically reinforcing said insulating cylinder.
 12. A pulse transformer for firing thyristors comprising a primary conductor conductively connected to a pulse generator, an insulating cylinder divided into a plurality of portions in axial direction, said primary conductor extending through said insulating cylinder, and a plurality of transformer units fitted around said primary conductor through said insulating cylinder therebetween, each of said transformer units including a magnetic iron core, a secondary winding wound on said iron core and connected to the gate of a thyristor and in insulating body for insulating said secondary winding the improvement comprising: a first conductive layer disposed in inner peripheral portion of said insulating cylinder and kept at an equipotential to that of said primary conductor; second conductive layers disposed in the inner peripheral portions of said insulating bodies of at least those transformer units which are positioned on the higher voltage side, facing said insulating cylinder; third conductive layers disposed in those surface portions of said insulating cylinder which face said second conductive layers, said second and third conductive layers being electrically connected respectively; said insulating cylinder portions having a converging and a diverging end surface; fourth conductive layers disposed in each facing end portion of said divided cylinder poRtions and electrostatically coupled to respective one of said third conductive layers, the end lines of said fourth conductive layers being offset in a radial direction to form an equipotential surface substantially perpendicular to the facing end surfaces of said insulating cylinder portion.
 13. A pulse transformer for firing thyristors as defined in claim 12, further comprising gaps between said primary conductor and said insulating cylinder, and between each pair of adjacent cylinder portions, said gaps being continuous, and a highly insulating medium filled in said gaps.
 14. A pulse transformer for firing thyristors as defined in claim 13, further comprising an upper and a lower vessel respectively attached at the upper and the lower end of said insulating cylinder and an insulating medium contained in said vessel, said gaps being continuous to said upper and lower vessel. 